Human breast cancer is the predominant malignancy and the leading cause of cancer death in women from Western society, as reported by Miller et al., (eds) BIOLOGY OF FEMALE CANCERS, 31-42 (CRC Press, 1997). According to recent estimates from the American Cancer Society, one in every eight U.S. women will have breast cancer and the disease will kill 43,500 women in 1998.
Several lines of evidence have strongly linked prolactin (PRL) to breast cancer development. Expression levels of prolactin receptors (PRLR) reportedly are higher in human breast cancer cells than in normal breast epithelial cells (Reynolds et al., 1997), or in surgically removed breast cancer tissues (Touraine, Martini P. et al., Increased Expression Of Prolactin Receptor Gene In Human Breast Tumors Versus Continguous Normal Breast Tissues, (Abstract) 79th Annual Meeting of Endocrine Society, p. 113, (1997)). PRLR levels in malignant breast tissue can be five-fold higher than in the surrounding normal tissue (see Touraine et al. (1997), supra, making the malignant cells highly sensitive to the stimulation by hPRL. Additionally, it has been suggested that one mechanism of the mitogenic action of estrogen in breast may influence the production and secretion of human prolactin (hPRL), since there is a positive correlation between PRLR, estrogen receptors or progesterone receptor levels (Sirbasku, 1978; Dixon and Lippman 1986; Lippman and Dickson, 1989). Taken together, these findings lead to a hypothesis that hPRL serves as an autocrine/paracrine growth factor that plays an important role in mammary carcinogenesis (Clevenger, et al., Am. J. Pathology, 146: 695-705 (1995); Ginsburg, E. et al., Cancer Res., 55: 2591-2595 (1995)).
An association between PRL expression and prostate disease has also been proposed in Wennbo et al., Endocrinol. 138: 4410-4415 (1997). PRL receptors are found in prostate tissue as reported Aragona et al., Endocrinol. 97: 677-684 (1975), and Leake et al., J. Endocrinol., 99: 321-328 (1983). In addition, PRL levels has observed that can increase with age (Hammond et al., Clin. Endocrinol., 7: 129-135 (1977), Vekemans et al., Br. Med. J. 4: 738-739 (1975)) coincident with the development of prostate hyperplasia. Transgenic mice overexpressing the PRL gene developed dramatic enlargement of the prostate gland. (see Wennbo et al. (1977), supra).
In view of its link to both breast and prostate cancer, PRL signaling represents an attractive target for therapeutic intervention. Heretofore, however, no suitable medicaments have been available for this purpose.
Inhibition of tumor angiogenesis has also been shown to hold great promise in treating cancer. Angiogenesis is a complex multi-step process that includes endothelial cell proliferation, migration, and differentiation, degradation of extracellular matrices, tube formation, and sprouting of new capillary branches (Tarui et al., 2001). Tumors often over-express several pro-angiogenic molecules, including members of fibroblast growth factor (FGF) and vascular endothelial growth factor families (VEGF, Kim et al., 1993; Cheng et al., 1996; Benjamin and Keshet 1997). Both vessel density and angiogenesis directly correlate with metastasis formation and prognosis (Vijayagopal et al., 1998; Guidi et al., 2000). Excessive angiogenesis is part of the pathology of cancer, and preventing angiogenesis in a tumor could effectively induce a dormant state in the tumor cells (Folkman, 1995; Hanahan and Folkman 1996). Blocking angiogenesis has demonstrated great promise as a therapeutic approach to treat or even eradicate cancer by cutting off its blood supply. Anti-angiogenesis therapy for cancer is effective because: (1) tumor growth is dependent on angiogenesis; (2) degree of angiogenesis is proportional to invasiveness of tumor; (3) tumor endothelial cells are qualitatively different from endothelial cells in adult non-neoplastics tissue; (4) endogenous inhibitors and stimulators of angiogenesis exist and have been isolated. Ryan and Wilding 2000. A number of unique biological effects make the angiogenesis inhibitors intriguing anticancer agents such as (1) acquired drug resistance may be less likely than with cytotoxic agents; tumor dormancy may be achieved through prolonged drug administration; (2) haematological toxicity is unlikely as often seen in chemotherapeutics; and (3) potential for synergy with cytotoxic agents.
Two important molecules that have the most promising affect on inhibiting angiogenesis are the soluble endogenous factors angiostatin and endostatin. Endostatin, a 20 kDa C-terminal fragment of collagen XVIII, was first characterized by O'Reilly et al. (1997) and has been reported to exhibit antiangiogenic and tumor-regressing activities (O'Reilly et al., 1997; Boehm et al., 1997). Angiostatin, a proteolytic fragment of plasminogen, has also been described to exert potent antiangiogenic and anti-tumor activities in a variety of tumor models (O'Reilly et al., 1994, 1996). The mechanisms by which endostatin and angiostatin inhibit angiogenesis are not known. Both endostatin and angiostatin are currently in early phase of clinical trials (see review by Herbst et al., 2001).
One of the most potent and specific angiogenic factors is VEGF (reviewed by Ferrara, 2001). VEGF and its high-affinity tyrosine kinase receptor Flk-1/KDR are central regulators of both physiological and pathological angiogenesis. The high expression level of VEGF and Flk-1 in the tumor endothelium indicates that this signal transduction system stimulates the proliferation and the survival of tumor vessels by a paracrine mechanism (Kim et al., 1993; Cheng 1996; Ferrara, 2001). Direct evidence for this hypothesis was provided by the inhibition of tumor growth in animal models by the application of VEGF neutralizing antibodies (Kim et al., 1993; Cheng 1996) or by the gene transfer of dominant negative Flk-1 receptor mutants (Millauer et al., 1994; 1996). Flk-1 expression is suppressed in adult endothelium, but is highly induced in the newly formed blood vessels in a variety of human tumors. Most recent studies using adenovirus as a delivery system to directly compare the efficacy of endostatin, angiostatin as well ligand binding ectodomains of VEGF receptors Flk-1 (Flk-1-BP) show that Flk-1-BP is a better angiogenesis inhibitor than either endostatin or angiostatin (Kuo et al., 2001).